In Floor heating system's Thermostat

Hi there
I would like some advice around the use of an infloor heating system's thermostat (Easy heat). 2 years ago we installed an infloor heating system with a sensor wire into the mortar underneath the tiles. The thermostat is a 7 day programmable one. To this day we have not done anything to the tiles that could have damaged the sensor wiring (such as drilling etc)

For the first year the thermostat and the floor worked fine. Great heat on your feet. However, we turned it off for the summer months and when we turned it back on the reading on the thermostat that indicates actual floor heat went up to 120F even through the tiles are stone cold and no heat can be felt. There is no heater or anything near the sensor causing it to perhaps to read that temperature instead and making the thermostat go to the maximum level the floor can be set to. Because that is the maximum level, the heating systeme won't come on.
Thinking perhaps the thermostat was defective we removed it and re-installed it thinking it would reset itself. No luck. So we replaced it with a new thermostat. This has not fixed the problem and the same is happening. At least it ruled out tha thermostat as a cause. We were able to get the floor to warm up, although the temperature was appr 86 or so, the thermostat required to be set higher in order for the heat to go up to a nice temperature. It seems to be that for some reason the thermostat receives a "message" that the floor is at the maximum hear even though that is not the case. So it is back up at 120 and no heat coming on as a result of that.
Does anyone have any idea what might be going on? The sensor has not been touched or the floor drilled in or anything. The fact that the heat did come on for a while tells me it all is working but somehow the temparture readings are not correct. All the components are easy heat. We called them but did not receive any call back after several attempts, let alone advice on what the issue could be.
Karen

The floor sensing probe is reading a temperature over 104F.Probe is likely too close or is touching a heating wire inside the mat.The probe must be relocated. Call your tile setter to reposition the sensor.

Since your thermostat is not displaying the above I would say your sensor is good also...

If you substituted a known good thermostat, there is something wrong with (or in the path to) the sensor. Sensors should be placed in a small accessible tube so they can be replaced. Do that if it turns out to be bad. Make sure there is good contact by using an appropriate heat conducting goo (that is a technical term ).

Are there any control electronics in the base attached to the electrical box on the wall. Does the thermostat directly control the power to the floor.

Where are the physical wires from the sensor attached to the thermostat or the base. They might be loose or corroded. Remove, clean with a very fine sand/emery paper.

If the thermostat is directly controlling the floor there is 120 V or 240 V present in that thermostat base. If you don't know how to handle that; call someone. You could get hurt and/or make pretty sparks until the breaker opens.

I would talk to the Easy Heat folks and get some ohm readings for the sensor. Then put an ohm meter on the sensor wires to see what you have. I would guess a short in the sensor wires. Could be a bad probe, or a problem with the connecting wires. The wires could be shorted where they enter the thermostat box (more likely if you have a metal box) or could be squeezed tight elsewhere. If you can find a physical short you can fix it, if it is in the sensor you will have to find a way to place a new one, or go with an air reading thermostat which, I personally do not feel is a good way to go.